MOS/SOI transistors are formed in islands of single-crystalline silicon on a substrate of insulating material, such as sapphire, with each island containing a single MOS transistor. The transistors are formed by epitaxially depositing a layer of single-crystalline silicon on the insulating substrate. A layer of silicon oxide is formed on the silicon layer. A layer of photoresist is coated over the silicon oxide layer and is defined, using standard photolithographic techniques, to leave portions of the photoresist over those areas of the silicon layer which are to remain. The uncovered portions of the silicon oxide layer and the portions of the silicon layer thereunder are then removed using a suitable etchant. A gate oxide is formed on the surface of each island, a conductive gate is formed over the gate oxide and source and drain regions are formed in the island either by diffusion or by ion implantation.
The silicon island is generally deposited on the (1102) oriented surface of the sapphire substrate so that the surface of the silicon layer has a (100) crystallographic orientation. Early in the development of SOI technology the island was formed by etching with potassium hydroxide using the slow etching (111) plane as the boundaries of the island. These (111) boundary sides sloped away from the top surface of the island. When forming a N-channel MOS transistor in such an island, the channel oxide and gate extended across not only the (100) top surface, but also across two opposed (111) boundary or side surfaces. This type of transistor was unstable because it had excessive edge leakage which resulted from the fact that an N-channel transistor formed along the (111) side surfaces of the island had a threshold voltage lower than that of a transistor formed along the (100) top surface. One attempt to overcome this problem was to make the channel regions of the transistors along the edge surfaces less conductive by implanting more conductivity modifiers in the side edges, as described in U.S. Pat. No. 3,890,632 to W. E. Ham et al., issued June 17, 1975, entitled "Stabilized Semiconductor Devices And Method of Making The Same". With the advent of reactive ion etching it became possible to etch vertically into a body of semiconductor material, i.e. perpendicular to the surface of the body. SOI islands were then formed with the side walls being vertical and extending along the (110) plane. These vertical side walls made the SOI/MOS transistor more stable, particularly when the transistors were subjected to high degrees of radiation. However, even greater stability would be desirable.